Evaporation of nitric acid



March 2-1, 1933. w. E. KIRST EVAiORATION 0F NITRIC ACID Filed Nov. 14, 1928 WILL/AM E 10257 lnvemor Patented Mar. 21, 1933 WELIAIE E. KIRST, 0F WOODBURY, NEW JERSEY, ASSIGNOR T0 I. DU POINT DE NEMOURS & COMPANY, OF WILMINGTON, DELAW'ABE, A CGRPGRATI ON 0E DELA- WARE EVAEORATEON OF NITRIC ACID Application filed November 14, 1928. Serial No.

This invention relates to an improved process of evaporation of nitric acid, and more particularly to a process of evaporation of nitric acid of various strengths by contacting the acid with a substantially dry surface maintained at temperatures substantially higher than heretofore employed, so as to minimize decomposition of the acid.

The vaporization of nitric acid has offered extreme difliculties largely because of the corrosive action of the acid on most materials of construction. Materials for this purpose have heretofore been limited to high silicon irons, such as Duriron, and

to fused silica. The siliconirons have the disadvantages of being exceedingly brittle, subject to thermal shock, and of being poorly adapted to large size apparatus which is essential for commercial scale production. lVith apparatus constructed of the foregoing materials, the rate of evaporation, expressed as pounds per square foot of active surface per hour, is slow. This is due primarily to the fact that only moderate temperatures can be used with safety and, hence, the temperature difference between the heated surface and the boiling point of the acid must necessarily be small, resulting in a low heat transfer and necessitating excessively large equipment. With the View of overcomin some of these disadvantages, chrome and c rome nickel steels have been introduced for nitric acid equipment. While these steels have proved to be much more satisfactory than materials heretofore used for this purpose, they exhibit a moderate corrosion when exposed to boiling acid in the liquid state.

This invention has as an object the vaporization of nitric acid of various strengths by contacting the acid with a heated surface. A further object of the invention is a proccss of vaporizing nitric acid on a hot, dry surface by maintaining the surface at a substantially higher temperature than has been. known in the vaporization of nitric acid. A still further object of the invention is the vaporization of nitric acid With a mini mum of decomposition of the acid. Anher object of the invention is to minimize the corrosion of the materials of construction employed in the vaporization of nitric acid. Other objects will appear as the description proceeds.

According to prior art practice, nitric acid has been heated to boiling by means of heating coils immersed in a pot of the liquid acid. lVhen steam is used as the heating medium, the temperature difi'erence between the heating medium and the boiling acid is low. For example, nitric acid boils at 2e8 F. and saturated steam under 15 pounds pressure has a temperature of 250.

F., while steam under 100 pounds pressure has a temperature of approximately 838 F. It is apparent, therefore, that at least steampressure above 15 pounds must be used, and even when steam pressure above 100 pounds is employed the temperature difference between the boiling liquid and the heating medium is only approximately F.

A suitable formula for calculating the square feet of surface necessary in a heating unit may be as follows:

Surface required Heat transmitted per hour ip B. t. u.

111 q- The coefiicient of heat transfer (B. t. 11. /sq.

ft./hr./F.) multiplied by temperature dif-' ference.

It is, therefore, apparent that to have an apparatus of minimum size which Will con-- L tain a minimum of heating surface, it is essential that the coefficient of heat transfer and the temperature difference must be as large a numerical figure as possible. The size of the numerical'figure for coefiicient of heat transfer is dependent mainly on the nature of the material used for construction,

the thickness of the metal, and the velocity A minimum.

tubes into which steam was admitted, has been found to be capable of evaporating only about seven pounds of veal: nitric acid per hour per square foot of surface.

I have found that chrome or chrome niclrel steels have approximately twice the resis ance to dry vapors of nitric acid that these steels have to the boilirn liquid acid. I have furthermore found that by spraying the acid to be evaporated in the form of a fine mist onto a hot surface maintained at a temperature substantially above tne normal boiling point of the acid, that the evaporating capacity per unit of heated area may be substantially increased.

No only this, but I have further found that the decomposition of the nitric acid during volatilization may be reduced to a This is accomplished by reason of the correspondingly large heat transfer that is permitted by maintaining a large temperature difference between the boiling point of the acid and the temperature of th heated surface. It has been found that by operating in the manner the entire working surface may be maintained practically dry, thus resulting in greatly lessening the rate of corrosion that takes place when liquid I nitric acid is present.

In my process I contemplate maintaining a temperature that will give the maximum volatilization without decomposition of the nitric acid. I have found that a suitable temperature for this purpose may be between 250 F. and 1G20 F., although it will be apparent that in some instances even higher temperatures may be desirable.

References to the use of chrome and chrome nickel steels are made in U. S. patents to Brearley, #1,197,256; Haynes, #1.- 299,404; Armstrong, #1322511; and Strauss, r b-1,339,378. These steels are much more satisfactory than any materials heretofore used for this purpose, but even these show a moderate corrosion rate when exposed to boiling acid in liquid state.

The accompanying drawing will illustrate one embodiment of my invention, but it will be understood that other embodiments exist and may be practiced. without departing from the spirit of my invention. In the drawing is shown more or diagrammatically an evaporating vessel D of suitable size and shape. The acid is supplied under pressure by the pipe A to the spray nozzle C. With the temperature maintained at the desired point, the acid feed is regulated so as to draw ed the vapors through B at practically their true boiling point.

Itwill be understood, of course. that other methods of acid distribution than the one herein illustrated may be used without departing from the spirit of the invention. For example, a rotating disc may be provided for throwing the acid onto the hot surface. As an example of another embodiment of my invention, the heat may be applied by means of electricity.

It will be apparent to those skilled in the art that my invention permits of various modifications without departing from the spirit thereof, and that I do not intend to belimited except as indicated in the appended claims.

I claim:

1. The process of vaporizing nitric acid, which comprises bringing the acid into contact with a dry surface of chromium steel maintained at a temperature substantially above the boiling point of the acid, and collecting the vapors of acid.

2. The process of vaporizing nitric acid, which comprises bringing the acid into contact with a dry surface of chromium steel maintained at a temperature substantially above the boiling point of the acid, and collecting approximately 99% of the acid free from decomposition.

3. The process of vaporizing nitric acid, which comprises bringing the acid into contact with a dry surface of chromium steel maintained at a temperature substantially above 250 F. and collecting the vapors of acid.

4. The process of vaporizing nitric acid, which comprises bringing the acid into contact with a dry surface of chromium steel maintained at a temperature substantially abov 250 F, and collecting approximately 99% of the nitric acid free from decomposition.

5. The process of vaporizing nitric acid, which comprises bringing the acid into contact with a surface mantained at a temperature above 250 F. such a rate that the surface is maintained dry, and removing at least 99% of the nitric acid free from decomposition.

6. The process of vaporizing nitric acid, which comprises spraying the acid on a surface maintained at a temperature above 250 F. at such a rate that the surface is maintained dry, and collecting the vapors of nitric acid.

7. The process of vaporizing nitric acid, which comprises spraying the acid on a surface maintained at a temperature above 250 F. at such a rate that the surface in maintained dry, and collecting approximately 99% of the nitric acid free from decomposition.

8. The process of vaporizing nitric acid, which comprises bringing the acid into contact with a surface of chromium steel maintained at a temperature between 250 F. and 1659 F, and collecting approximately 99% of the nitric acid free from decomposition.

In testimony whereof. I my signature.

IVILLIAM E. KIRST. 

